Plastic materials are lightweight, highly tough and easy to be dyed, and therefore are widely used recently for various types of optical materials, particularly eyeglass lenses. Optical materials are required to have, as properties, low specific gravity, high transparency and low yellowness, and as optical properties, high refractive index and high Abbe number, and high heat resistance, high strength, etc. A high refractive index allows an optical material to be miniaturized, and a high Abbe number reduces the chromatic aberration of an optical material. High strength facilitates secondary processing and is important in terms of safety, etc. As the technique of simultaneously achieving high refractive index and high Abbe number as optical properties and high heat resistance, use of an episulfide compound is well known.
Meanwhile, when producing a cured product of an episulfide compound having a high refractive index and a high Abbe number including eyeglass lenses, a resin composition containing the episulfide compound is injected into each of desired molds, then put into an oven or the like to be polymerized and cured, and released from each mold to obtain the product. This is because, in the case of the resin composition containing the compound, the amount of heat generated by ring-opening polymerization of episulfide groups is very large, and it is required to use a mold in a size which is as small as possible for facilitating control of heat generation. When heat cannot be effectively removed, for example, when the thickness of the thinnest portion of the internal space of the mold is more than about 2 cm, heat generated by polymerization cannot be completely removed and the temperature of the composition sharply increases, and it may cause yellowing of an optical material obtained, and in some cases, it may result in rapid polymerization to cause a decomposition reaction of the composition. Further, even when a small mold is used, for example, in the case where a lens-shaped optical material is thick, striae tend to be easily generated due to convection caused by heat generation at the time of polymerization, and it is difficult to reduce striae to the level that a product obtained can be used as an optical material.
Thus, when using a mold in a size which is as small as possible, it becomes relatively easier to control heat removal, and it is possible to produce a cured product suitable for optical materials including eyeglass lenses, but there are drawbacks: (1) it is required to prepare a required number of molds corresponding to the number of products to be produced; and (2) since it is required to inject the resin composition into each of molds one by one, the production cost is high and productivity is low. Therefore, if a cured product of an episulfide-based resin having a large size and free of yellowing and striae appropriate for optical materials can be produced, by subjecting the cured product to cutting work to make an optical material, the production cost can be significantly reduced. In addition, it is possible to provide a thick optical material such as a convex lens having an unprecedented large size from the cured product of the episulfide-based resin having a large size, and it is very useful. For this reason, it has been desired to develop a method for the production thereof (see Patent Documents 1-4).